In CRM you have the ability to create what are called business units. Now when you install Microsoft Dynamics CRM you will get a primary business unit created by default and typically it is the name of your company. This is known as the primary business unit and is the parent or root business unit in Microsoft Dynamics CRM.

You must note that the parent business unit cannot be deleted, deactivated or changed. It can be renamed, and that is pretty much it. However, you can group and build other business units that are called child business units off of the root business unit. Much like an hierarchical structure.

Business units and security roles literally go hand in hand. They allow you to separate various levels of access to specified groups in your organization. Users are assigned to business units and security can be restricted to records that are in that business unit group.

For example you could have a structure with a primary business unit and also a child business unit for sales, a child business unit for service, and a  lastly a child business unit for Marketing.

This would allow you to use the entities even custom ones in CRM that have been created, but only have access to records created in your business unit. If you had a merger or recent acquisition you could also use a business unit to provide a level of separation in your dataset to different groups of users and that might be another case depending on your example that child business units would come in handy.